Musnad Ahmad 759:1

Waki' narrated to us, Ibn Abi Layla narrated to us, from Thabit al-Bunani, from Abd al-Rahman ibn Abi Layla, from Abu Layla, who said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) reciting in a prayer that was not an obligatory one. He passed by a mention of Paradise and the Fire, so he said: "I seek refuge in Allah from the Fire. Woe—or alas—to the people of the Fire."